In the present description the designation dozer or bulldozer is used for both the specific earth moving equipment known as a ‘dozer’ in the trade and for similar earth moving equipment having a height adjustable blade at the front.
In the present description the designation IMU is used for an inertial sensor with one gyroscope only.
In the present description the designation pivot-to-surface distance is used for the fixed distance between the surface that the dozer or similar earth moving equipment is moving on and the pivots that are attachments for support arms for the cutting blade and around which the aggregate constituted of supporting arms and cutting blade performs a rotary movement under the influence of hydraulic cylinders. In practice a dozer will under most circumstances move on a surface that has been subjected to the action of the blade and which hence is close to the design surface in its properties.
A dozer with a blade is well-known for use as earthmoving equipment in shaping surfaces with respect to elevation and inclination, such as in the profiling of roads. Another way of expressing it is that a dozer performs a function of preparing a surface defined by the line of the cutting edge of the blade when it is carried forward by the dozer. Manual operation of such equipment requires both great skill and previous accurate positioning of markers (reference points) to guide the height and tilt adjustments of the blade. Various systems comprising calculators are known that provide input to apparatus that will inform the operator of the adjustments needed from instant to instant. The blade is carried on supporting arms fitted on the chassis of the dozer at pivot points by means of bearings that permit a lifting and lowering of the blade, which hence performs a movement in an arc of a circle. This rotating motion can be converted into a vertical movement by knowledge of the machine geometry. The cutting edge must be controlled to a high precision, but overshoot, residual oscillation, and stepwise changes must usually be avoided in dozer work. The need for working at a high speed is mainly relevant when the work is in straight horizontal lines or straight planes. This type of work constitutes the majority of the cases. If an automatic control is used, height and angle information is used as the target value in a feedback loop controlling the hydraulics of the dozer.
The supporting arms for the dozer blade are moved by means of hydraulic cylinders that are supplied with hydraulic liquid under pressure via valves that are controlled manually, or as in the present apparatus, by means of electromagnetic valves that are activated under the control of the apparatus. The viscosity of the fluid and the supply provided by the valves are both temperature and working pressure dependent, and these are essentially non-linear relationships that can, however, be made to work inside a negative feedback loop. All the well-known problems with feedback loops are obviously also present here. This may be counteracted in well-known ways by the use of PID controllers, but the system may thereby become too slow for a speed that is within the capabilities for earth-moving of the dozer. However in order to utilise the speed optimally, special corrective means are required.
In order to obtain a target surface, absolute references are required. The reference information is required on a continuous basis and with a rate of updating that is commensurate with the speed of automatic operation. Virtual references are obtained by means of GNSS systems, in which a receiver processes signals from several transmitting satellites in order to calculate a three-dimensional position of the antenna. When this antenna is placed on a pole on the blade its vertical position at the time of measurement is provided with sufficient accuracy, however, if the blade is moving this is only a historical fact, due to latencies caused by amongst other things calculations and data transmission. The vertical noise level is dependent on a number of different factors, such as the number of simultaneous signals received, the position of each satellite, and the distance to the base station. It will also increase at high latitudes due to the orbits of the satellites. The update rate is typically high but this height reference type has a significant noise component and a non-negligible delay associated with it.
Another type of reference is obtained by means of a stationary active device placed at a location with accurate coordinates. This device, sometimes termed an Automatic Total Station (ATS), optically measures the distance and angle to a retro-reflecting device mounted on a pole and transmits this information to the calculator that applies trigonometric calculations in order to determine the position of the blade in space. The update rate is low and the latency large, however it is very accurate.
A further type of reference is obtained by means of a rotating or scanning laser beam from stationary equipment placed at a location with accurate coordinates. A receiver on a pole comprising several receiving elements provides information of the vertical position with respect to the laser plane. If it is desired to obtain a plane surface from the work of the dozer, the operator has merely to maintain the height or vary it according to a pre-determined rule. The update rate is typically quite high and the latency and noise level very low, however at long distances between the receiver and the rotating laser device the noise level increases—especially in windy conditions.
The first limiting factor with current systems with regards to performance is caused by drawbacks of the absolute height sensor in use. This height sensor on the blade provides input to the control system with an irregular, infrequent rate, which is delayed in time and further has a noise component. The degree of these different disadvantages depends on the absolute height sensor type in use.
A second limiting factor is that the hydraulic system, which is included in the control loop, has an unknown non-linearity and an unknown delay that may also change with time and temperature. Hence modelling the hydraulic system is in practice not possible, since the complex relationship between the control signal and the blade motion cannot be determined.
These two limiting factors have a significant influence on the performance of the control loop and these factors are the main bottlenecks in prior art with regard to operating speed and surface smoothness.